Cellular telephones have undergone a dramatic market growth in the past few years. These existing systems utilize analog FM modulation techniques. In order to transmit dam, landline modem signals are transmitted over cellular systems by using the cellular telephone as a twisted pair replacement. The trend in the industry is toward replacing the analog FM system with digital modulation and transmission means, e.g. GSM, IS54, JDC. IS54 is a so-called dual mode system in which the existing analog and the new digital modulation have to coexist. Thus one portable handset will have to be capable of communicating using either the analog or the digital cellular signals. Other types of signals are being transmitted over the cellular network, as described in U.S. Pat. No. 4,914,651. This diversity of signals requires terminal equipment to include a receiver which is capable of dealing with multiple modulation techniques or "protocols."
FIG. 1 shows the spectrum of an aliased input signal X which is to be converted from an analog signal to a digital signal. Such an aliased input signal X includes a plurality of aliased copies of a signal of interest, and may be obtained, for example, from a sample and hold circuit, such as are known in the prior art or as is described in co-pending U.S. patent application Ser. No. 07/936,361 on an invention entitled "A Direct Conversion Receiver for Multiple Protocols". It is desired to select only that aliased copy which is centered around .omega..sub.0 while rejecting the other aliased copies of the input signal. If a linear converter is utilized the quantization noise level N.sub.L (.omega.) of the conversion is determined by the number of bits contained in the converter, as shown in FIG. 2. Using an oversampled converter allows for simpler construction of the converter as well as shaping of the quantization noise N.sub.3 (.omega.) (as also shown in FIG. 2) to allow for a higher precision conversion.
FIG. 3 shows a block diagram of a bandpass sigma delta converter as described in Ping, L., Dept. of Electrical and Electronics Engineering, University of Melbourne, Parkville, Victoria 3052, Austrailia, IEEE, pp. 1645-1648. As shown in Ping's equation (1), ##EQU1## output signal Y is a function of the gains of the two input paths, a and b. If a and b are -1 and 1, respectively, the system equation reduces to Ping's equation (2), ##EQU2## which shows that input signal X is digitized without filtering and the quantization noise N(z) is shaped by the transfer function H1 of the bandpass sigma delta converter.
Ping also describes a cascaded system which is used to achieve higher orders of shaping of the input signal X, thereby shaping both the desired information signal and its associated noise.
One problem with such prior art techniques is the lack of selectivity of input signal X with respect to undesired adjacent channel signals. While the bulk of the selectivity may be performed in subsequent stages of an overall receiver design such as that shown in the above-mentioned co-pending U.S. patent application Ser. No. 07/936,361, and more specifically in the novel digital decimation filter described in co-pending U.S. patent application Ser. No. 07/934,746 on an invention entitled "A BandPass Decimation Filter Suitable for Multiple Protocols", the lack of selectivity of prior art bandpass sigma delta converters is undesirable and increases the difficulty of establishing appropriate gain levels within the sigma delta converter. While it is desirable to set the gain levels within the sigma delta converter to accommodate the largest expected signal, this is not adequately achieved in the prior art due to lack of selectivity with respect to adjacent channel signals. The nonselective nature of prior art designs prevents optimal signal to noise results for the conversion of the desired input signal X.
An example of a typical communications signal is shown in FIG. 4. In this figure there is a strong undesired adjacent channel signal B which is to be rejected in order to accept and analog to digital convert the desired information signal A centered at .omega..sub.0. The prior art system does not adequately reject the undesired adjacent channel signal B, but rather leaves this task to the subsequent decimation filter. Because of this non-rejection of adjacent channel signal B, the dynamic range of the signals in such prior art sigma delta converters are determined in large part by the undesirable adjacent channel signal B and not solely by the desired information signal A. Thus accuracy of the conversion from analog to digital of desired information signal A is dependent on the difference in levels between it and undesired adjacent channel signal B. This can be seen in FIG. 5, which plots the signal to noise ratio of an analog to digital conversion against the signal strength of the input signal to an oversampled converter. As shown in the example of FIG. 5, amplification is set such that the amplified input signal has a signal amplitude of 2.5 volts. In the prior art example of FIG. 4, in which there is a large adjacent channel signal, the sum of the desired information signal A and the undesired adjacent channel signal B is amplified to 2.5 volts. This necessarily means that the amplitude of the amplified desired information signal A alone is much less than 2.5 volts. In the example of FIG. 5, if the amplified desired information signal A has an amplitude of 25 mv, its signal to noise ratio is only 20 dB--significantly less than the 85 dB signal to noise ratio of the composite input signal X having such components A and B.
Another problem in the prior art is the loss of dynamic range in the later stages of a series of cascaded converters. For the same reasons as stated above the dynamic range of the signal is critical to maintaining a high accuracy conversion. Ping's design requires attenuation of the signal entering the second stage, signal X2 in Ping's paper, by a factor of four before it enters the second stage of the converter. This decreases the possible accuracy of the converter.
Furthermore, Ping's prior art system utilizes the biquad formulation for implementation of transfer functions H1 and H2. This filter formation is widely known to have difficulties with coefficient sensitivity and dynamic range of intermediate nodes.